1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, and to a method of preparing the photoreceptor, and an image forming apparatus, an image forming method and a process cartridge using the photoreceptor.
2. Discussion of the Background
Recently, information-processing systems using an electrophotographic method are making remarkable progress. In particular, laser printers and digital copiers which record data with light by changing the data into digital signals make remarkable improvements in their printing qualities and reliabilities. Further, technologies used in these printers and copiers are applied to laser printers and digital copiers capable of printing full-color images with high-speed printing technologies. Because of these reasons, photoreceptors are required to produce high-quality images while maintaining high durability.
Photoreceptors using organic photosensitive materials are widely used for laser printers and digital copiers due to their cost, productivity and non-polluting properties. As the organic electrophotographic photoreceptors, the photoreceptors including photoconductive resin typified by poly-N-vinylcarbazole(PVK); charge transfer complex type photoreceptors typified by PVK-TNF(2,4,7-trinitrofluorenon); pigment dispersion type photoreceptors typified by phthalocyanine-binder; and functionally-separated photoreceptors typified by combinations of a charge generation material (CGM) with a charge transport material (CTM) are known.
Among these various photoreceptors, the photoreceptors using organic photosensitive materials are mostly used as functionally-separated photoreceptors because of having good sensitivity and durability, wherein the charge generation materials and charge transport materials can individually be designed at a molecular level.
However, the organic photoreceptor having a photosensitive layer mainly formed of a low-molecular-weight CTM and an inactive polymer is typically soft and has a disadvantage of being easily abraded due to mechanical stress of a developing system and a cleaning system when repeatedly used in the electrophotographic process. When abraded, the potential and photosensitivity of the photoreceptor tend to deteriorate, resulting in background fouling due to a scratch on the surface thereof and deterioration of density and quality of the resultant images. Therefore, abrasion resistance of the organic photoreceptor has been an important subject. Further, recently, in accordance with speeding up of the printing speed and downsizing of an image forming apparatus, the photoreceptor has to have a smaller diameter, and durability becomes increasingly important.
A conventional mechanism to form an electrostatic latent image in the multi-layered photoreceptor is as follows:
the photoreceptor is charged and irradiated with light;
the light passes through the charge transport layer (CTL) and is absorbed by the CGM in the charge generation layer (CGL) to generate a charge;
the charge is injected into the CTL at an interface of the CGL and the CTL;
and the charge moves in the CTL by an electric field and neutralizes the charge on the surface of the photoreceptor to form an electrostatic latent image.
To improve the abrasion resistance of the photoreceptor, methods of imparting lubricity to the photosensitive layer, hardening the photosensitive layer, including a filler therein and using a high-molecular-weight CTM instead of a low-molecular-weight CTM are widely known. However, another problem occurs when these methods are used to prevent the abrasion of the photoreceptor. Namely, an oxidized gas such as ozone and NOx arising due to use conditions or environment, adheres to the surface of the photosensitive layer and decreases the surface resistance thereof, resulting in a problem such as blurring of the resultant images. So far, such a problem has been avoided to some extent because the material causing the blurred images are gradually scraped off in accordance with the abrasion of the photosensitive layer. However, in order to comply with the above-mentioned recent demand for higher sensitivity and durability of the photoreceptor, a new technique has to be imparted thereto. In order to decrease an influence of the material causing the blurred images, there is a method of equipping the photoreceptor with a heater, which is a large drawback for downsizing the apparatus and decreasing the electric power consumption. In addition, a method of including an additive such as an antioxidant in the photosensitive layer is effective, but since a simple additive does not have photoconductivity, including a large amount thereof in the photosensitive layer causes problems such as deterioration of the sensitivity and increase of residual potential of the resultant photoreceptor.
As mentioned above, the electrophotographic photoreceptor having less abrasion by being imparted with abrasion resistance or a process design around thereof inevitably produces blurred and low-resolution images, and it is difficult to have both high durability and high quality of the resultant images. This is because high surface resistance of the photosensitive layer is preferable for preventing blurred images and low surface resistance thereof is preferable for preventing an increase of residual potential.
Japanese Published Unexamined Patent Application No. 2000-231204 discloses a method of including at least a compound having a dialkylamino group in a photosensitive layer to solve the above-mentioned problem such as blurring of the resultant images due to a blur generating material such as an oxidizing gas. However, the compound has an effect on image quality after repeated use, but the resultant photoreceptor does not have high sensitivity and cannot comply with high speed printing because of having low charge transportability. Therefore, an additional amount thereof has a limit, and a method of combining the compound with a CTM to increase sensitivity and repeated use stability of the resultant photoreceptor is disclosed therein.
On the other hand, it is described that a stilbene compound having a dialkylamino group disclosed in Japanese Published Unexamined Patent Application No. 60-196768 and Japanese Patent No. 2884353 has an effect on the blurred images due to the oxidizing gas on page 37 of Konica Technical Report Vol. 13 written by Itami, et al. and published in 2000. However, since the compound has a substituted dialkylamino group having a strong mesomeric effect (+M effect) at a resonance portion in its triarylamine structure, which is a charge transport site, total ionization potential is extremely small.
Therefore, the compound has a critical defect of being quite difficult to use practically because charge retainability of a photosensitive layer in which the compound is used alone as a CTM largely deteriorates from the beginning or after repeated use. In addition, even when the above-mentioned stilbene compound is used together with other CTMs as it is in the present invention, the compound has a considerably smaller ionization potential than the other CTMs and becomes a trap site against a charge transport, and therefore, the resultant photoreceptor has quite a low sensitivity and a large residual potential.
Japanese Published Unexamined Patent Application No. 2004-258408 discloses a method of reducing hole trap sites to decrease a residual potential, wherein a compound having an alkylamino group has an oxidation potential not less than that of a CTM by a specific level. However, when the surface of a photoreceptor having higher durability is not refreshed or a oxidizing material such as NOx, having a very high concentration, is present, the method does not satisfy all the requirements such as abrasion resistance, and prevention of blurred images and residual potential.
Because of these reasons, a need exists for an electrophotographic photoreceptor producing images without the deterioration of image quality such as blurred images and increases of residual potential even after repeatedly used for long periods or in an environment including large amounts of an oxidizing gas.